In order to understand the molecular mechanisms underlying paramyxovirus pathogenesis, we have tried to clarify the role of genes encoding glycoproteins as well as to establish the method for the recovery of infectious viruses from cloned viral genomes. Through the characterization of mumps virus stains exhibiting different fusion activity, we have shown that the amino acid at position 262 was crucial in terms of determining the extent of induction of cell fusion. We have also immunized mice with plasmid DNA containing the F gene of mumps virus and found that the immunized mice produced antibody against the F protein. Although it is difficult to obtain anti-F antibody that can neutralize viruses, this immunization protocol was quite efficient to induce neutralizing anti-F antibody. Moreover we have succeeded to produce infectious measles viruses by transfection of plasmid DNA containing cDNA corresponding the entire measles virus genome according to the method established by Billter et al. This experience seems extremely important in establishing the reverse genetics for mumps virus.